1. Field of the Invention
The invention relates to mixers used to mix liquids and solids. More particularly, the invention relates to a stirrer for use with a planetary mixer and a planetary mixer incorporating the stirrer.
2. State of the Art
Planetary mixers can be described as mixers that have at least one stirrer (or mixing blade) that revolves (or orbits) in a mixing vessel (or tank) about a central axis while simultaneously revolving on its own axis. Since planetary mixers move the stirrer or stirrers through all areas of the mixing vessel, they are especially useful when mixing high viscosity mixtures which cannot be adequately mixed with a mixer having a fixed stirrer. Planetary mixers are usually oriented vertically with one, two, or three stirrers revolving about a vertical axis. The stirrers are mounted on a drive mechanism which includes means for lifting the stirrers out of the mixing vessel.
The most commonly known planetary mixer is the type that is typically found in kitchens for mixing dough and various food products, having a single stirrer and somewhat hemispherical bottom mixing bowl. Small machines of this type, such as those made by KitchenAid(trademark), are usually found in residential kitchens as well as some commercial kitchens. Larger machines such as those made by Hobart(trademark) are usually found in commercial kitchens. There are other manufacturers of these machines, but most of the designs are similar. These mixers are commonly made in sizes for processing from 1 gallon through 35 gallons. These mixers are generally not capable of vacuum mixing and they are generally not used in industrial manufacturing or with high viscosity mixtures.
The most common type of industrial planetary mixer has multiple stirrers and a flat bottom mixing vessel. These machines are typically used to process materials such as; adhesives, sealants, coatings, foods, cosmetics, pharmaceuticals, plastics, plastisols, composites, metal powders, magnetic coatings, precious metals, ceramics, ceramic metal products, bulk molding compounds, castable materials, dental composites, etc. They are used in any industry where difficult to mix materials need to be processed. They are used for liquid/liquid mixing, liquid/solid mixing and solid/solid mixing. They are used in laboratories, pilot plants and production plants. They are commonly made in sizes for processing from xc2xc pint through 500 gallons. The most typical of these have two stirrers, although there are some designs that have three stirrers. See., e.g., U.S. Pat. No. 4,697,929, the complete disclosure of which is hereby incorporated herein by reference. The most common stirrer design is a substantially rectangular element such as that shown in prior art FIGS. 1-3.
As shown in FIGS. 1-3, the stirrer 10 has an upper central mounting collar 12 having a vertical bore 14 and two horizontal intersecting bores 16, 18 for set screws (not shown) and/or drive pins or keys (not shown) for transmitting torque to the stirrer. A pair of arms 20, 22 extend outwardly and downwardly from the collar 12. Vertical stirring elements 24, 26 depend from the arms 20, 22 and are coupled to each other at their bottoms by a cross bar 28.
As mentioned above, the stirring element 10 is coupled to a drive shaft in a mixing assembly which causes the stirrer 10 to rotate about the axis of bore 14 and also causes the stirrer to orbit about another axis, typically the central axis of the mixing assembly. The top speed of the outer edge of the stirring elements 24, 26 can range from less than 5 feet per minute to over 600 feet per minute. This is the combined speed of the rotation of the stirrer on its own axis and the rotation of the stirrer assembly as it orbits about the central axis of the mixer.
Stirrers of the type shown in prior art FIGS. 1-3 have certain drawbacks. Primarily, they do not have good top to bottom mixing ability. It is therefore necessary to increase the mixing time in order to get a homogeneous mix.
Wetting out powders in liquids as well as adding powders to higher viscosity materials usually requires adding a small amount of powder at a time and waiting until this is mixed in, before adding more. This is because of the poor top to bottom mixing action and because adding too much powder typically brings the level of the powder up higher than the top of the stirrers.
Inserting the rectangular stirrers into high viscosity material as well as lifting them out requires a great deal of force because of the crossbar at the bottom of the stirrers. This requires a heavy duty lifting mechanism to overcome these forces thereby increasing the cost of the machine.
The bottom crossbar typically lifts a large amount of material out of the mixing vessel when the stirrers are raised if the viscosity is high. The material must be scraped off of the stirrers by hand. This is very labor intensive and difficult especially on larger machines.
The rectangular stirrers have stagnant areas on the back (or trailing edges) and the sides of the stirrers that can harbor materials that do not get mixed into the batch. This is because these stirrers have flat areas on the backs of the stirring elements. See, e.g., 24a, 26a in prior art FIG. 3.
The rectangular stirrers also allow material to form balls or columns that get knocked around rather than being mixed when the viscosity increases. If the mixing is allowed to continue past this point, the material quite often gets pushed up out of the mixing vessel and into the area around the gearbox. When this happens, the mixing operation is usually considered a failure, and the mixer must be stopped and the material dug out from the gearbox area by hand.
The rectangular stirrers also cause torque spikes as the stirrers come near each other, and as the stirrers come near the side walls of the mixing vessel. This is because the entire length of the stirring elements of the stirrers (from top to bottom) come near each other at one time and the entire length of the stirring elements (from top to bottom) come near the side of the mixing vessel at one time. These torque spikes are one factor that can limit the ability of the mixer to mix high viscosity materials. Overcoming this problem requires a heavier duty drive system, which increases the cost of the machine.
The crossbar (28 in FIG. 1) at the bottom of a rectangular stirrer is needed primarily for strengthening the stirrer rather than for any mixing action. The crossbars cause undue strain on the mixing machine because of the high side loads that they produce at the farthest point away from the gearbox as well as the additional torque required to rotate the bottom crossbars in high viscosity materials. Their presence is another factor that limits the ability of the mixer to mix high viscosity materials. Overcoming this problem requires a heavier duty drive system, which increases the cost of the machine.
In addition, the arms 20, 22 are normally located below the top of the mixing vessel and are either slightly above or even at the level of the full mixing capacity of the machine. The presence of these arms at this height is a contributing factor to material climbing up out of the mixing vessel. It also limits the amount of powder that can be added at one time and be quickly mixed into the batch.
In order to mix high viscosity materials with a double planetary mixer with rectangular stirrers, it is a common practice to greatly reduce the batch size being mixed, which dramatically reduces the production capacity of the machine. It is a common practice when mixing difficult materials to mix only xc2xd or even ⅓ of the full working capacity of the mixer. Even by reducing the batch size, problems are still frequently encountered with the material forming balls or columns and not mixing properly.
There are many design alternatives to the rectangular stirrer, each of which overcomes some of the problems discussed above. However, no stirrer design overcomes all of the problems discussed above.
It is therefore an object of the invention to provide a non-rectangular stirrer for use with a planetary mixer.
It is also an object of the invention to provide a planetary mixer incorporating the stirrer(s) of the invention, especially for use with double planetary mixers of the type having flat bottom mixing vessels and planetary dispensing type mixers.
It is another object of the invention to provide a stirrer which has improved top to bottom turnover.
It is still another object of the invention to provide stirrers that will perform well at wetting out powders that are introduced to the top of a liquid even if larger amounts of powder are added at one time than would normally be added with rectangular stirrers.
It is also an object of the invention to provide stirrers that can be inserted into and removed from high viscosity materials with less force than is required on rectangular stirrers.
It is another object of the invention to provide stirrers that will not lift out a large amount of material when the stirrers are raised after mixing high viscosity materials.
It is still another object of the invention to provide stirrers that will greatly reduce the amount of material that is harbored on the back (or trailing edges) of the stirrers.
It is also an object of the invention to provide stirrers that will reduce the tendency of material to form balls or columns rather than being mixed.
It is another object of the invention to provide stirrers that will greatly reduce the tendency of material to climb up out of the mixing vessel thereby causing a failure of the mixing process.
It is still another object of the invention to provide stirrers that will produce lower torque spikes than rectangular stirrers.
It is also an object of the invention to provide stirrers that will not put unnecessary strain on the gearbox or shafts due to bottom cross bars.
It is another object of the invention to provide stirrers that will not tend to push material up out of the mixing vessel as the material comes in contact with low mounted top cross bars.
It is still another object of the invention to provide stirrers that will typically not require a greatly reduced batch size in order to be able to mix high viscosity materials.
It is also an object of the invention to allow materials of a higher viscosity to be mixed on a given mixer than if the same mixer was fitted with rectangular stirrers.
In accord with these objects which will be discussed in detail below, the stirrer of the present invention includes an upper central hub with two outwardly extending horizontal arms and two helical stirring elements, one depending from each arm. According to the presently preferred embodiments, the cross sectional area of the stirring elements is large enough that no bottom crossbar is necessary. The pitch angle of each stirring element is preferably constant from top to bottom. The leading side of the stirring elements has a flat surface whereas the trailing side is curved. The flat surface increases in size moving toward the bottom of the stirring element. Comparison tests performed with the stirrer of the invention and a conventional rectangular stirrer demonstrated that the stirrers of the invention tolerated a viscosity approximately four times the viscosity at which the conventional stirrers could no longer mix. Visual observation made during the testing indicated a much better top to bottom mixing action as well as much better ability to push powder down into the material. It was also observed that the lifting mechanism was better able to insert and remove the new stirrers into and out of the material than with the standard stirrers and there was very little material lifted out of the mixing vessel when the stirrers were raised. Another test using water soluble dye demonstrated that the shape of the back of the new stirrers eliminated the dead areas that exist with rectangular stirrers.
Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.